Transcript PowerPoints

Types of interactions
-
0
+
- Competition Amensalism
Predation,
parasitism
0 Amensalism
Commensalism
+ Predation,
parasitism
Commensalism
Mutualism
Consumer-Resource Interactions
• All life forms are both consumers and victims
of consumers.
• Consumer-resource interactions organize
biological communities into consumer chains
(food chains):
– consumers benefit at the expense of their
resources
– populations are controlled from below by
resources and from above by consumers
– The relative importance of top-down versus
bottom up control of populations is an
important focus of ecological research
Some Definitions
• Predators catch individuals and consume
them, removing them from the prey
population.
• Parasites consume parts of a living prey
organism, or host:
– parasites may be external or internal
– a parasite may negatively affect the host
but does not directly remove it from the
population
More Definitions
Parasitoids consume the living tissues of their
hosts, eventually killing them:
– parasitoids combine traits of parasites and
predators
Herbivores eat whole plants or parts of plants:
– may act as predators (eating whole plants)
or as parasites (eating parts of plants):
• grazers eat grasses and herbaceous
vegetation
• browsers eat woody vegetation
Predation
Theory: Lotka-Volterra Equations.
P = Predator population size
V = Prey resource
dV/dt = rV – σPV
dV/dt = 0  P=r/σ
Geometric increase of prey (resource) in absence of
predator; subtract predation, where σ is catching
efficiency.
dP/dt = βVP – qP
dP/dt = 0  V=q/β
Geometric decrease of predators in absence of prey;
Predation loss of prey corrected for assimilation
efficiency, or " β ".
Solution:
Limit cycles (periodic solutions) such that
• Species can coexist, but
• Random walk to extinction,
• No interaction of prey with food supply
(& no time lags),
• Predator mortality independent of
prey density.
Testing the theory -- Gause’s Paramecia
Testing the theory -- Huffaker’s oranges
Case studies – Opuntia and Cactoblastis
Cactoblastis chronology
• 1839 Opuntia stricta in pot to Australia f/Texas or
Florida
• 1900 10,000,000 acres
• 1925 60,000,000 acres (i.e. area twice size NC)
increasing at 1,000,000 acres per year. Too dense to
walk, 3-6' high. Sheep would not eat, horses could
not traverse.
• Cactoblastis - northern Argentina
2750 eggs in 1925; 2x106 eggs out in 19 locations
• 1930-31 Opuntia ravaged, mostly back to grass
• 1932-33 Opuntia recovered some
• 1935-40 Cactoblastis recovered and expanded
• >1940 Only scattered Opuntia plants remained
Klamath weed and Chrysolina quadrigemina
L-V Assumptions
• Growth of victim population is limited
only by predation (exponential growth)
• Predator is a specialist on victim
(starves in absence of victims)
• Individual predators can consume an
infinite number of victims
• Predator and victim encounter one other
randomly in a homogeneous environment
Carrying capacity
Functional response
Keystone predators – Piaster and Mytillus